With regard to an engine of a vehicle such as a car, improvement of a warm-up performance of the engine and improvement of a fuel efficiency by moving the engine at an appropriate temperature and the like have been examined. This was achieved by providing, besides a main passage which circulates coolant between the engine and a radiator, a bypass passage which bypasses the radiator and returns the coolant directly to the engine. A coolant control valve was provided in the main passage, and an opening degree of this coolant control valve was controlled according to a coolant temperature and other values. An amount of the coolant that is flowing in the main passage and cooled off by the radiator was also controlled. For example, at the engine starting or the like, when the coolant temperature is low, warm-up of the engine is promoted by blocking the main passage and returning the coolant from the bypass passage directly to the engine without allowing the coolant to pass through the radiator. Also, for example, for controlling the temperature of the coolant so as to optimize combustion of fuel in the engine after the warm-up, opening and closing (the opening degree) of the coolant control valve is controlled.
As such a coolant control valve, for example, a rotary valve that is driven by a stepping motor or the like, and a thermostatic valve that is moved according to a temperature are examined. Incidentally, as the thermostatic valve, a thermostat, a thermowax or the like that is displaced according to a temperature is adopted, and this displacement according to the temperature opens and closes the valve.
Herein, if, by any chance, the coolant control valve stops its operation in the closed state, the coolant is circulated in the engine via the bypass passage without being cooled off by the radiator, whereby the coolant temperature is increased. If the engine is operated as it is, the engine may become overheated. It has been suggested to circulate the coolant towards the radiator by a valve (other than the coolant control valve) which is provided in a thermal protection device that is operated when the temperature of the coolant is increased when the coolant control valve stops its operation in the closed state or the like (for example, see Patent Literature 1).
That is, the valve that is provided in the thermal protection device works as a fail-safe mechanism. Incidentally, the valve provided in the thermal protection device is a valve which adopts a device that is displaced according to a temperature, for example, a thermostat, a thermowax, shape-memory alloy, a combination of alloy that is melted at a preset temperature with a spring or the like. When the coolant temperature is increased to the preset temperature or higher, the device is displaced according to the temperature so as to open the valve.